Buried in Mt. Tom: For nearly a century, reservoir has served Eau Claire

Nov. 24—EAU CLAIRE — Buried inside of Mt. Tom for nearly a century now, a giant reservoir remains vital to getting water to most of Eau Claire's population.

At the top of the mount in the North Side Hill neighborhood there are benches for those weary from making the uphill trek, a few small utility shacks and security fencing around a grassy area.

This protected toupee of turf is an indication of what lies beneath it — a 3.2-million-gallon reinforced cement vessel that since 1925 has been providing water pressure to a growing city.

"It's very important. It's the central piece," said Mark Nelson, Eau Claire's water plant supervisor.

The height of Mt. Tom — its peak is at an elevation of 1,025, which is more than 100 feet taller than streets and neighborhood blocks around it — is why the reservoir is as useful today as it was in the 1920s.

"Mount Tom keeps the pressure," said Cole Cloutier, the city's utilities engineer.

A map of the city's water pressure zones shows Mt. Tom is still generating the necessary pressure to serve most of Eau Claire. On the outer edges of the city where homes and businesses were built in hilly areas as Eau Claire developed, other reservoirs, water towers and pumping equipment are there to make sure those zones have the right water pressure.

Should the Mt. Tom reservoir go offline for some reason, including the once-in-a-decade inspection when the reservoir is emptied, there are other water tanks in Eau Claire that shoulder the workload. The city's largest water tank — a 5.6-million-gallon standpipe off Rudolph Road — can back-up Mt. Tom.

The Mt. Tom reservoir is the oldest water tank in the city's system still in use. Others, including water towers and standpipes, were built between 1952 and 1998, according to an annual report of the Eau Claire Municipal Water Utility.

But the reservoir's condition belies its older age.

"It's actually quite amazing how good a shape it is in," Nelson said.

When it was drained for a full inspection in 2017, the interior proved to be surprisingly well preserved. Photos from that inspection show a white paper label from the 1920s construction crew still firmly adhered to the ceiling and printing on it legible.

Though some of the pipes leading to the reservoir have been replaced over the decades and more modern measuring devices installed, the concrete structure itself has not required work since it was built.

That's in contrast to metal water towers and standpipes that will need repainting and repairs or replacements of their interior coatings. In the next five years, the city is estimating it will have two $1.4 million upkeep projects at other reservoirs and water towers. But there's $0 in projects planned for the Mt. Tom reservoir during that time.

"It's an amazing water tank because it doesn't require a lot of maintenance," Nelson said. "It just lies there and does its job."

The reservoir passed its latest inspection with flying colors.

Back when it began

In the early 20th century as Eau Claire grew, there had been discussion about improving water service and fire protection.

Adding the reservoir to Mt. Tom was seen as a key part of doing that.

The project was a long time in the planning, as evidenced by the March 1918 date on original designs for the reservoir drafted by city engineer J.T. Hurd.

All the planning came to a head in 1924.

An inspection by the National Board of Fire Underwriters done in April of that year found that Eau Claire's water pressure dropped significantly when fire hydrants were opened, according to an article published in the Eau Claire Leader newspaper.

"Speaking generally, the proposed new reservoir on Mt. Tom should be built as soon as possible as the city has no reserve storage at the present time," Harvey T. Munn, a hydraulic inspector wrote in his report.

In June 1924, the Eau Claire City Council authorized $165,000 in bonds for several projects to improve the city's waterworks. Constructing the Mt. Tom reservoir and a long access tunnel for its piping were budgeted for $68,000 from the bonds.

A. Larson & Co. — a partnership between Andrew Larson and L.G. Arnold — was selected as contractor for the project.

Arnold's grandson, Dean Arnold, 74, has chronicled numerous projects that his family's company has been involved in the Chippewa Valley, including the reservoir.

He created a video slideshow of the Mt. Tom reservoir earlier this year, which is on display on a TV at the Chippewa Valley Museum.

Himself a retired civil engineer living in Chicago, Dean Arnold is planning to continue creating presentations on the many building and infrastructure projects that his grandfather helped create.

"These projects do have a lasting legacy," Dean Arnold said in a phone interview with the Leader-Telegram.

Reservoir building begins

Logging and clearing the ground at the top of Mt. Tom was underway by Sept. 5, based on an Eau Claire Leader article. Then came the excavation with a steam shovel, scooping shale off the mount so it could be hauled away in horse-drawn carts.

An October 1924 article proclaimed that dynamite blasting, machinery and horse teams were "... making a most perceptible dent in the face of nature on the top of Mt. Tom."

Within two months, the excavation had been completed with 14,000 cubic yards of shale and sandstone removed, the Eau Claire Leader reported in November 1924.

Next came pouring of the concrete floor of the reservoir — a foot-thick slab of concrete reinforced with metal.

"The people of Eau Claire have little cause for anxiety as to any danger which may arise from the storage of water in Mt. Tom, when the imposing strength and solidity of the structure is taken into consideration," the newspaper proclaimed on Nov. 12, 1924.

During the same month, work finished on the 220-foot-long tunnel bored into the side of Mt. Tom. This is where large pipes were laid to connect the reservoir to the city's water system.

As winter approached, the reservoir continued to take shape and provided an interesting sight to those who saw a photo of it in the Dec. 7, 2024, edition of the Eau Claire Leader. The reservoir wall had been poured by then, creating a deep, circular structure atop Mt. Tom.

"No, this is not the amphitheater of the ancient coliseum at Rome, nor is it a boxing arena or an indoor race track in the process of construction," the newspaper playfully wrote.

A cold snap around Christmas with a few days of below-zero temperatures put the project on a brief pause, but crews returned to the work site on Dec. 26, 1924.

By Jan. 21, 1925, the last of the wooden forms for the columns and roof were complete, but the weather was not yet warm enough to pour concrete. But that later changed and before the end of March, the last bucket of concrete had been poured to finish the roof.

While that meant the project was practically done, newspaper reports throughout the rest of spring documented the finishing details. That included removing the wooden forms, adding earth around the reservoir's wall and more piping work to get it connected.

An August 1925 article on a fire at the Gillette Rubber Co. factory noted the Mt. Tom reservoir was in service at that point.

Mentions of the reservoir crop up in following months as more of the city's water system is connected to it.

An Oct. 29 article titled "Big reservoir on Mount Tom shows worth" explained how the water pressure used for fighting fires had been doubled since the reservoir went into service.

And on Dec. 13, 1925 — a Sunday — the Mt. Tom reservoir quietly became the sole source for Eau Claire's water for most of that day. The city's main supply was shut off for 19 hours to install new, larger piping, so the reservoir took over during that time. During that time, the reservoir's water level dropped just 3 feet with 2.75 million gallons remaining in the tank, according to an Eau Claire Leader article. Waterworks officials stated they were satisfied with the reservoir's performance and it was a good test for how it would come in handy during an emergency.

Construction of the Mt. Tom reservoir even came in under budget by about $10,000 — money the city used on additional water projects.

Going inside

Nelson and Cloutier are among the few people to go inside the reservoir since it was completed in 1925.

Both were in there while an independent engineer inspected it five years ago.

"You feel like you're in a big cave," Nelson recalled.

The round reservoir is 160 feet across and its walls are 22 1/2 feet tall, according to designs for it. The floor slopes down toward its center where there is a 12-inch-wide pipe connecting it to the rest of the city's water supply.

Leading up to the inspection, water was drained first through equipment at the city's water plant by lowering the overall level in the system. Remaining water is then piped out of the reservoir down a temporary chute on the side of Mt. Tom, ending up in a nearby storm sewer.

One worker is lowered down through a hatch to secure a ladder for the others to follow down into the darkened interior.

The walls are black from decades of manganese — a naturally occurring mineral the city works to filter out of its water due to discoloration. Even with the brightest flashlights they bring with them, Cloutier said it was still hard to see in there due to the dark surroundings.

"It's really neat though," he said.

Photos from the inspection show the rows of dark columns faintly illuminated by the workers' flashlights. At the base of some columns are names the 1920s workers etched into the cement as it dried.

While the modern-day workers were in the empty reservoir, they kept their voices soft.

"When you do these tank inspections, you don't talk loud," Nelson said.

Even if you're talking to someone on the opposite side, he said a normal conversational volume will do. Anything louder and the echo inside the tank will make it hard to understand what you're trying to say.

After the inspection is over, workers get out, seal up the reservoir tight and refill it again — leaving it to continue doing its job unnoticed by passersby.

Newspaper articles of the 1920s even remarked that the reservoir's aesthetic impact on Mt. Tom was only the loss of a few trees and the ground being about four feet taller than it was before.

Though unseen, what's buried there continues to make a difference in the lives of Eau Claire residents, whether they realize it or not.